Many signal and/or data transmission devices between two distant points separated by a determined distance are already known in the art. In particular, many systems using connections by coaxial cables or transmission lines called "buses" are known. However, with these standard systems, and when the task is to transmit video signals, it is generally necessary that said signals are frequency or amplitude modulated and that they do not remain in the baseband if one wishes to avoid a quick degradation of said signals, which is all the more important as the diameter of the coaxial cables used is smaller. With cables of large diameter, the degradation of the signal is less but the installation is costly and above all less convenient. The problem is forming passages in the walls through which the large diameter cables are to pass to provide connections with the equipment at said locations. For cables of smaller diameter, a frequency or amplitude modulation of the signals is always necessary, thereby making necessary the use of a modulator and then of a demodulator, introducing some distortion and limitations. Moreover, some of said systems designed for long range transmission of signals from the output of a tape recorder or those which are to be applied to the input of a television set of a known type, and using the demodulator present in the standard television sets, are incompatible with the use of simple visualization monitors.
The present invention is a device allowing the transmission of at least one signal, such as but not limited to a a video signal, maintained in the baseband without modulation. The device is reliable, simple to produce and allows transmitting a signal without distrubing it, and without this signal disturbing the environment, or without or itself being disturbed by said environment. The device also has a wide pass-band, excellent linearity and a signal/noise ratio remaining satisfactory across said band.
More particularly, the invention is directed to a transmission device which allows transferring signals corresponding to information of any nature and of the type which can be found in many industrial as well as domestic applications. Thus, in the case of hotels and communities for example, the present device allows advantageously communication from a given central station, or more generally between several stations. The present device can transmit various telephonic, radio, or television messages, information relating to security, the supervision and the eventual control of inlets and outlets, the change of codes or door locks, and the usage of some services, wake-up calls, etc. In such applications, it is obvious that when the account of information that can be transmitted by leads of small section becomes more important, the multiplication of unaesthetic and bulky cables and the disadvantages due to setting them in place increases in proportion. The limitation to the twin-wire line of the invention, each having a diameter which can be notably less than 1 mm, allows easy passage of the lines underneath washboards, between floor planks, underneath doors, and underneath carpets. This is a great advantage particularly in homes or hotels.
On the other hand, in the video and transmission of television images field, it is known that said images come more and more frequently from various sources, notably tape recorders, cameras, converters of signals transmitted by coaxial cables or optic fiber beams, video disc readers, and television sets. The problem is establishing communication upon request from each of said sources at will with other receivers or monitors at the disposal of the users. Outside even the hotel or community field, for example a simple domestic usage, one can easily imagine the possibility of establishing inside the same home a transmission assembly, possibly connected with an audiophonic assembly, allowing a remote transmission from one room to the other of the television images accompanied by sound signals and also remotely controlling the sources of said signals.
The present invention is therefore directed to a multi-input transmission device for conveying signals, such as video signals, but possibly of an other nature, which can be transmitted according to the standards of any system (SECAM, PAL, NTSC, D2 MAC PAQUET . . . ) and are from any appropriate source (a receiver for a radio transmitter, a receiver antenna for transmissions from satellites, cables, etc. The device avoids the use of transmission coaxial that are liable to introduce bulky and costly interference in the various signals transmitted, especially if the cables are of a great length.
Transmission systems using two leads of small diameter are certainly already known. Thus, FR-A-2 584 555 refers to a wide band signal transmission device in a network, possibly of video-communication with a star-shaped structure, where the video information can be remotely transmitted to a plurality of distinct receiving sets. In such systems, the signals to be transmitted are adapted in an input equipment and transmitted to an output equipment via a balanced and symmetrical twin-wire line through a switching matrix including M lines and N columns.
The matrix is associated with an address memory allowing selection of the chosen switching address. Finally in said systems, the transmitted signals are filtered in the input equipment so as to separate the low and high frequency components, each of which being the object of a transmission on the twin-wire line according to a different mode. Regarding the high frequency component of the signal, it is sent in a differential mode through a transformer, the two components going, in the output equipment, through a summation device for reconstituting the signal. Such a device is costly, complex and risks introducing signal distortion with a poor signal/noise ratio, above all if the frequency band is made relatively wider.
With respect to the prior art thus shown, the present invention has the advantage of a very simple device, allowing transmitting in optimal conditions any signals received at its input, whatever the standard used. With the present device it is not necessary to separate the high and low frequency signals in the signal spectrum, instead said signal can be immediately remotely transmitted in its whole extent by the two leads of the line equipped with symmetrical transformers, due to an appropriate adaptation and a balancing of the line avoiding the radiation of said line.
To this effect, the signal transmission device, transmitting signals between at least one transmitter assembly and at least one receiver assembly by means of a twin-wire connecting line of small diameter, equipped at each end with a symmetrical transformer and powered by at least one current generator, is characterized in that the current generator outputs symmetrically to the line and is controlled by the signals to be transmitted. Said signals generally have a frequency between 50 Hz and 30 MHz and the line is balanced at each end by an impedance equal to its characteristic impedance.
Advantageously, the two identical wires are not shielded and each have a diameter less than or equal to 1 mm, it being understood here that the invention has its essential advantages in the use of leads of small diameter but larger diameter wires can be used. Moreover, although it is obvious that the wires of the line have preferentially a circular cross-section, they could also be flat, notably in the shape of tapes placed on an insulating support or imbedded in said support, without departing from the scope of the invention.
Preferably also, the symmetrical transformers fitting out each end of the two wire lines have magnetic core with each winding mounted in series with one of the wires of the line. According to a particular embodiment, the windings of the symmetrical transformers are themselves made of the line wires. These transformers strongly reduce line noise without damping the high frequency components of the transmitted signals.
According to another feature of the invention, each transmitting assembly includes two current generators, one controlled by a signal and the other by the inverse of that same signal and delivering symmetrically on one and the other of the line wires, said generators being mounted in series with a matching impedance unit having a value which is equal to the line characteristic impedance. Preferably, the impedance unit in series with the generators includes resistors and a capacitor adapted to separate the continuous component from the signals to be transmitted. Moreover, the matching impedance unit is advantageously formed by means of two equal impedance units mounted in series and with the node between the two units connected to the ground via a capacitor. Likewise, in each receiver assembly, the matching impedance unit of the line is mounted in parallel on said line and is as such made of two equal impedance units mounted in series with a medium point connected to the ground.
According to another advantageous feature of the invention, each receiver assembly includes two current generators supplying the power supply current to said assembly. Moreover, and according to another feature, the receiver assembly includes a complementary current generator, operating symmetrically, with its voltage controlled by modulated signals, distinct from those controlling the generators of the transmitter assembly, whereby said signals can be in particular remote controlled signals circulating on the twin-wire line in a direction reverse to that of the signals received from the transmitter.